1. Technical Field
The present application relates to a tool for machining a workpiece, such as a milling tool.
2. Background Information
Such a tool is known from U.S. Pat. No. 5,667,428. The machining of workpieces usually requires two different machining operations. First, a rough-machining of the workpiece, also called roughing, is effected, with which high metal-removal rates shall be achieved. For roughing, mostly diamond, ceramic or carbide cutting edges are used. After the rough-machining operation, the machined areas have a rough surface which must as a rule be reworked. This is done by the finishing or grinding operation, e.g. by means of a grinding wheel with sharp-edged diamond ICBN abrasive coatings. Contrary to roughing, only low metal-removal rates are reached in this case.
One of the differences between the two machining operations is the fact that they are usually effected at different rotational speeds of the tools, a far higher rotational speed being used for roughing. While for machining cast iron, cutting speeds up to 1000 meters per minute are used for roughing, the cutting speed for finishing amounts to only approx. 300 meters per minute. With a rotating tool having cutting inserts arranged on the periphery, the peripheral speed corresponds to the cutting speed. Another difference is the fact that grinding usually requires a coolant for cooling, i.e. a wet machining operation is effected. Contrary to this, roughing requires a dry machining operation, because the carbide or ceramic cutting edges are not very sensitive to thermal shocks.
It has already been tried to carry out the two machining operations by means of one tool. U.S. Pat. No. 4,993,891, for example, discloses a surface milling cutter for milling and grinding of a workpiece. On the outer edge of the peripheral side of the cylindrical surface milling cutter, a multitude of cutting inserts is mounted. Inwardly of the cutting inserts, on the underside, square grinding elements with abrasive surfaces are arranged in a circular row. The orientation of the grinding elements, which remove the traces of rough-machining and smooth the surface of the workpiece, is of importance in this surface milling cutter. The grinding elements have a small positive axial angle of inclination and a small lead angle, so that their radially inner edge is slightly lifted off from the grinding surface. Furthermore, the grinding elements have a radial angle of inclination and are, therefore, twisted in such a way that they offer a larger grinding surface. In operation, first of all the cutting inserts engage with the workpiece and mill it, followed by the grinding elements, which grind the workpiece. In this embodiment, the grinding elements are firmly mounted on the tool and have the same cutting speed as the cutting inserts, which amounts to approximately more than 1000 meters per minute. At this cutting speed, without coolant, the grinding elements pick up very quickly material removed from the workpiece, so that they have to be exchanged far more frequently than the cutting inserts.